GRAδIENT stands for GRAmophone DIsc Enhanced Noise Treatment and is a unique noise-reduction process for shellac-era needle-drops introduced for the first time in Stereo Lab.

Seventy-eight RPM records were made of shellac. But a slate filler was added to the pure shellac to make it hard enough that the records could support multiple playings with a tone-arm weight measured in ounces! It is these particles of slate which contribute the harsh, surface-noise that we all associate with pre-vinyl records.

The development of GRAδIENT followed much work on Fourier-based noise-removal, all of which proved pretty disappointing. In our experience, by the time Fourier Transform based noise-reduction produces noticeable and worthwhile improvement in noise, it also introduces perceivable and unpleasant audio artefacts which include: "birdies" and a hollow "ringing" quality to the programme.

The GRAδIENT solution was empirically derived and was tested in extensive listening tests. It uses a unique algorithm which radically reduces surface noise without removing wanted high-frequency information. GRAδIENT also acts to reduce tracing distortion and correlated-noise due to groove damage. This is especially clear in the example of the needle-drop of the metal mother example given below.

GRAδIENT is selected in the Phono EQ preferences dialogue where the choice may be retained to use alternately a simple 7kHz low-pass filter, or indeed to opt for no surface-noise removal at all.

A demonstration of GRAδIENT is given here.

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In this file the first minute of so of a recording of the Peer Gynt Suite by Grieg (LPO with Beecham - Columbia LX838) is noise reduced in 4 ways. In the first section, the recording is processed via Stereo Lab's de-click/ de-crackle algorithm. The second example adds the standard, needle-scratch filter. The third track substitutes the standard filter with GRAδIENT. Finally, for reference, the de-clicked version is processed by a well known (and expensive!) FFT-based noise reduction. We think you'll agree with us that GRAδIENT outstrips the other techniques.